What are Defi cryptocurrencies?

Could you elaborate on the concept of DeFi cryptocurrencies, please? I'm interested in understanding what makes them unique and how they operate within the broader cryptocurrency ecosystem. Specifically, I'd like to know about their decentralized nature, the services they provide, and how they aim to disrupt traditional financial institutions. Additionally, I'm curious about the risks involved in investing in DeFi cryptocurrencies and the potential returns they offer. Your insights would be greatly appreciated.